What Causes Fecal Incontinence With Dry Flaky Stools?

Hello, My almost-6 yr old (aprox 45 lbs) daughter is experiencing fecal incontinence. She has a constant oozing stool in her underpants along with dry flaky bits of stool. she eats a diet high in fiber, low in refined sugar/cabs, low in dairy (only yogurt a couple times/week), etc. we assume the sneaky poo is due to constipation. it is getting worse and she hasn t had a BM in 5 days! (at least not a successful one. She has tried many times with limited output ) she is drinking a minimum of 6 (8oz) servings of fluid per day with at least 4 of the 6 being pure water and the rest being diluted juice (and she s had 16 oz of diluted gatorade in the past 24 hrs). she hasn t ever taken laxatives until two days ago. i gave her one chewable pedialax (magnesium sulfate, i think) before bed and one in the morning with no result. then i gave her three tablets last night before bed with no result. I gave her 4 tabs almost 5 hours ago along with 16 oz of water on an empty stomach. since then she s only had one egg white, a large bowl of watermelon, a glass of diluted apple juice, and a cup of warm peppermint tea with honey. She does not seem ill other than slight lethargy. though she hasn t had a BM in 5 days, before that she had three pretty typical ones in a period of 36 hrs. she does have a history of digestive difficulty and fecal withholding but never until now has she experienced this soiling. it is dry and fluid at the same time and is occurring constantly.